Practice question

Intestinal epithelium renews every 4-5 days from Lgr5 positive crypt base columnar stem cells residing at crypt base intermingled with Paneth cells providing niche signals. Niche constitutes Paneth cells supplying Wnt3, EGF and Notch ligand Dll4, mesenchymal telocytes and Foxl1 fibroblasts producing R-spondins, Gremlin antagonists and BMP inhibitors generating Wnt-high BMP-low environment maintaining self-renewal and repressing differentiation. EphB-Ephrin gradients position cells along crypt-villus axis regulating sorting. Upon Wnt inhibition stem cells differentiate into goblet, enteroendocrine and absorptive enterocytes migrating upward toward villus tip. Hematopoietic niche, epidermal niche and neural niche reside elsewhere regulating respective stem populations.